If you plan to buy the new iPhone 6s or 6s Plus, T-Mobile has a deal for you. It's a bit complicated, but you ultimately save $124 on the cost of the phone if you buy from T-Mo instead of Apple and then keep it for 18 months.

Apple will sell you the 16GB iPhone 6s for $27 a month, for 24 months, while T-Mobile charges $20 a month (plus sales tax, of course) over 18 months, plus one final payment of $164. That comes out to $648 if you buy from Apple and $524 from T-Mobile.

The larger iPhone 6s Plus costs about $4 more a month on either company's plan. T-Mobile hasn't announced price information for the more expensive 32GB and 64GB iPhones, nor has it said how long this "introductory" offer will last.

Neither plan includes insurance. If you want it, T-Mobile charges an $8 a month, and Apple adds $5.41 to the bill — but it also throws in AppleCare+, which extends the hardware warranty to 2 years.

In addition to the iPhone price cut, T-Mobile announced a new satisfaction guarantee for customers who buy these new devices through its Jump! On Demand offering. If a customer isn't happy within the first 30 days, T-Mobile will refund the full purchase price. After the first 30 days, and during the life of the contract, unhappy customers can return their phones, pay nothing more and receive a refund equivalent to one month's service fees.

Future phone upgrades through Apple, T-Mobile

If you buy from Apple, you can swap your iPhone for a new one after a year. (You can find more details on the company's new iPhone Upgrade Program on its website.) Apple usually refreshes its phones in the early fall, so if you buy right now, you'll be able to upgrade to the iPhone 7 next September.

T-Mobile's offer is a bit more complicated, and customers have three options.

At the end of 18 months, 16GB iPhone 6s owners will have paid $360 in lease payments. Add another $164 to cover the outstanding balance, and they can walk away with an unlocked iPhone. If they don't want to keep the phone, they simply give it back to T-Mobile without paying the $164.

If a customer wants to upgrade, they hand over the phone to T-Mobile without paying any more money on the old phone, and then start the payment cycle all over. The option is valid anytime, so if you purchase the iPhone 6s Plus and decide after a month that it's too big, you could then "upgrade" to the iPhone 6s with no penalty and still be able to upgrade to the iPhone 7 a year from now. And you can upgrade as many as three times in one year, if you purchase the phone via the Jump! program.

There's also another benefit: You don't have to upgrade to another iPhone. You could, for example, decide you want a Samsung Galaxy instead.

T-Mobile plans to begin taking preorders for the new iPhones at midnight on Sept. 12. The company expects — but does not guarantee — customers will receive their orders by Sept. 25. (AT&T, Verizon and Sprint have not yet announced their iPhone prices.)

San Francisco journalist Bill Snyder covers business and technology. He writes regularly for CIO.com, Stanford's Graduate School of Business, and the Haas School of Business at the University of California at Berkeley.